If I decide to apply for CS in LAS but later change my mind and want to do engineering, how hard would it be to switch the school but stick to the same major within the first year.
You probably will apply for cross campus transfer before even declaring major, or you may waste some time. First, you apply to LSA, not LAS. This would be a critical mistake if on your why this school assay. Second, the intended major on application is just for a survey purpose. If you plan to transfer, you need to register the course accordingly.